In humans there are five RecQ helicases whereas in simpler organisms there is only one. It is of great interest to explore how these RecQ helicases function individually and together and whether there is redundancy of helicase function or whether each has unique individual roles. There may be circumstances where they can function in a synergistic manner and this would be informative about whether these different RecQ helicases interact with each other and also about the biological importance of certain processes. We and other groups find that the human RecQ helicases participate in DNA repair, and more specifically in certain subpathways of DNA repair. Three of the RecQ helicases are deficient in disorders of segmental premature aging: Werner syndrome, Bloom syndrome and Rothmund Thomson syndrome. Much less is known about the Rothmund-Thomson syndrome protein, RECQL4, and RECQL5 than about the other human RecQ helicases. We find that the RECQL4 protein participates in DNA repair of double and single strand breaks and that cells from individuals with this condition are defective in DNA repair. Using real time imaging, we have shown that RECQL4 is recruited to sites of double strand breaks and that its retention kinetics are different than WRN or BLM. Additionally, we mapped the domain of RECQL4 necessary for DNA damage recruitment. Biochemically, we are characterizing the RECQL4 protein and while its helicase function is in many ways similar to WRN and BLM helicases, there are also significant differences. We have shown that RECQL4 has a very limited substrate range and that its helicase activity can be seen on short fork DNA substrates but not on long substrates. We are also exploring potential protein interactions and protein complexes that RECQL4 participates in. We find that RECQL4 can interact with and stimulate the core BER proteins APE1, pol &#61538;and FEN1. We are continuing to define and explore the catalytic activities of RECQL4 and potential protein:protein interaction partners. RECQL5 is another member of the RecQ family for which little information is known. We have previously reported that WRN, BLM and RECQL4 can stimulate core base excision repair proteins. Thus we evaluated whether RECQL5 could also stimulate and interact with BER proteins. Indeed, we found that RECQL5 could interact with and stimulate FEN1 incision reactions. We also evaluated RECQL5s ability to interact with and stimulate APE1 or pol &#61538;&#61484;&#61472;however RECQL5 did not stimulate these proteins. Therefore within BER, we are beginning to define which activities are shared among the RecQ helicases and which functions are unique to the individual RecQ helicases.